9800 GTX+ SLI Question

i've got a question... in my pc now i have 2 BFG 9800GTX+ OC 1GB (sli) and i found another 9800 gtx+ ( 512mb) for a dirt cheap price and i want to know if i could do TRI SLI by adding the 9800 gtx 512mb. I'm not sure if it will work since both mine are 1gb and the other one is a 512mb. let me know what u think.